Electrophotographic process using corona discharge current of an asymmetrical wave form

ABSTRACT

An electrophotographic process comprising the steps of (a) applying a first charge by means of direct current corona discharge to an overcoated photosensitive sheet consisting of a conductive base, a photoconductive layer and a surface insulating layer; (b) applying a second charge to the so charged sensitive sheet by means of a corona discharge current of asymmetrical wave form in which the discharge current of a polarity opposite that of the first charge is greater than that of the same polarity and the degree of asymmetry is sufficient to satisfy the specified requirements, and simultaneously exposing the sensitive sheet through or original having an image pattern of light and shadow: and (c) thereafter exposing the photosensitive sheet to an uniform illumination over the entire surface to form a latent electrostatic image of the light area and a latent electrostatic image of the dark area, which are of opposite polarity to each other.

United States Patent 1 1 Matsumoto eta].

Dec/29, 1970 Japan 45 121544 ELECTROPHOTOGRAPHIC PROCESS USING CORONA DISCHARGE CURRENT OF AN ASYMMETRICAL WAVE FORM Inventors: Shoji Matsumoto, Neyagawa; v

Noboru Yonaha, l-Iirakata; Tat'suo Aizawa, Osaka,"all of Japan Mita Industrial Company, Ltd.,

Osaka, Japan Filed: Dec, 28, 1971 Appl. No.: 212,898

Assignee:

Foreign Application Priority Data US. Cl. 96/] R, 96/1 C, 355/3,

317/262 A', 3l7/262 AE Int. Cl...; 603g 13/22 Field of Search 96/] R, l C, l E;

References Cited UNITED STATES PATENTS 3,438,706 4/1969 Tanaka et al. 96 1 RX 3,457,070 7/1969 Watanabe et al. 96/] R x 3,536,483 10/1970 Watanabe et al. 96/1 R 3,615,395 10 1971 Yamaji et al. 96 1 R/ 3,655,369 4 1972 Kino shita 96 1 R Pfimmy-Eltdmirief-Roland E. Martin, Jr,

Attorney-Leonard W. Sherman et al.

[5 7] ABSTRACT An electrophotographic process comprising the stepsv of (a) applying a first chargeby means of direct current corona discharge to an overcoated photosensitive sheet consisting of a conductive base, a photoconduetive layer and a surface insulating layer; (b) applying a second charge to the so charged sensitive sheet by meansof a corona discharge current of asymmetrical wave form in which the discharge current of a polarity opposite that of the first charge is greater than that of the same polarity and the degree of asymmetry is sufficient to satisfy the specified requirements, and simultaneously exposing the sensitive sheet through or original having an image'pattern of light and shadow: and (c) thereafter exposing the photosensitive sheet to an uniform illumination over the entire surface to form a latent electrostatic image of the light area and alatent electrostatic image of the dark area, which are of opposite polarity to each other.

5 Claims, 22 Drawing Figures 'This invention relates to a new electrophotographic' 1 On the other in Japanese Pat. application publication NO. 23910/1967a method is disclosed which consists of the steps of applying the aforesaid oven I coated photosensitive-member with adirect current process. More specifically,-the'invention relatesfto an electrophoto'graphic process which can form ron the surface of an overcoated photosensitive membera latent electrostatic charge image of a given polarity of the light area-and at the same time a latent electrostatic charge image of the opposite polarity of the dark area.

'An e'lectrophotographic process for forming a latent electrostatic image corresponding to an original copy by simultaneously charging and imagewise exposing an overcoated photosensitive member, e.g., a threelayered overcoated photosensitive member consisting of a conductive base, afphotoconductive layer and a surfacev insulating layer, is known, for example from, Japanese Pat. application publication No. 4121/ 1961.

In Japanese 1 "Pat; application publication No. 2478/1968 an improvementof the, foregoingelectro photographic process, isprovided comprising a method which comprises impressing the aforesaid overcoated photosensitive member with a direct current voltage of specific polarity by means of a corona discharge, then exposing this photosensitive member to a light image tive charge. Next, as a result of a second charge and theexposure of an optical image, the light area and the dark' area of the photosensitive member are both charged with,'. for example,a negative charge Now,

when the photosensitive member is exposed to uniform illumination over the entire'surface area by means of activating radiation, the surfacejpotential 'of the dark voltage of specific polarity by means of a corona dis:

' charge, and next applying this photosensitive member with-an altemative corona discharge while exposing it at the same time to an optical image and thereafter exposingthe photosensiive member to a uniform illumination over the entire surface area by means of an activating radiation..While this method possesses the advantage that the instability of the negative corona discharge can be avoided, the reversing of the polarity of thecharges of the latent electrostatic images of the light and dark area is theoretically impossible.

ln electrophoto graphy the reversion of the polarity of thecharges of the images of the light and dark areas is very desirable to increase the contrast'of the image formed as well as for solving the soiling of the background .by prevention of the so-called fogging during the development operationjFu'rther, if the polarities of thesurface potential of the two are also reversed, the developing toner particles in adhering to the latent electrostatic image of opposite, polarity during the development operation also receive an electrostatic repulsive force from the latent electrostatic image area of while applying th photosensitive member with adirect i the opposite polarity to result m an effluent accomplishment of the development. lnaddition, it becomes possibleto freely obtain either a positive or negative image by a suitable choice of the developing toner par-' charge current ofan asymmetric wave form in which are'ajdecays. Thus, it becomes possible to increase the contrast of the potentials between the latent etectrostatic images of the light area and thedark area: In this caseit is theoretically possible to makethe surfacepotential of the dark area positive is the process is so conducted that the positive charge by means of the first charge is not comple'lely neutralized by means of the negative charge of the second'charge. However, the direct current. negative corona discharge is not asstable as the-positive corona discharge or'the alternating current corona discharge. Therefore, it-is practically 'impossible to effect the charging'uniformly especially with a low charge density (see, forexample, Japanese 1 Pat. application publication No. 21432/1966). Further,

even one attempts to control the extent of the charge by means of the charging time, this is practically impossible to do, since the charging must be terminated at a period when thecharging potential'charge's very violently. As a result, from the standpoint of forming a stable and clear image, it is nearly impossible to operate the corona current of a polarity opposite that of the first charging is greater than .that of the same polarity,

images whose polarities are opposite to'each other.

Thus, an electrophotographic process is provided according to the present invention, which process comprises the steps of applying a first charge by means of a direct current corona 'discharge'of a specific polarity to "either a three-layered overcoated' photosensitive member consisting of a conductive base,'aphotoconductive layer and a surface insulating layer or a fourlayered overcoated photosensitive member consisting of a conductive base, aphotoconductive layer, a sur- .face insulating layer and an intermediateinsulating layer disposed either within the photoconductive layer or between the photoconductive layer and the conduc tive base; applying to the so chargedphotosensitive member a second charge by means of a corona dis charge current of an asymmetrical wave form in which the discharge current of a polarity opposite that of the first charge is greater than that of the same polarity and simultaneously exposing the photosensitive member through an original having an image pattern of light and shadow, the degree of asymmetry of the corona discharge current of an asymmetrical wave form being in this case sufficient to transform both the polarities of the charged potentials of the light and dark areas of the photosensitive member to opposite polarities and also sufficient to form during the uniform illumination over the entire surface area of the photosensitive member a latent electrostatic image of the dark area of the same polarity as that of the first charge and a latentelectrostatic image of the'light area of an opposite polarity to that of the first charge; and thereafter. exposing the photosensitive member to a uniform illumination over the entire surface area by means of an activating radiation to form a latent electrostatic light image and a latent electrostatic dark image, which are of opposite polarity to each other. I I t I The inventioniwill now be fully described with reference'to the accompanying drawings, whereinz FIGS 1,-a to 1-0 are views illustrating the sectional construction of the photosensitive member used in the process of the present invention;

FIGS. 2 -a to 2-e are views explaining the several steps of the process of the present invention; FIGS. 3-a to 3-0 are charts illustrating the changes in the potential of the photosensitive member as a whole, the surface of the insulating layer and at the interface of the insulating layer and the photoconductive layer; FIGS. 4-41 and 4-b are respectively a wiring diagram of the normal alternating current corona discharge apparatus and a graph showing the wave form of the symmetric discharge current obtained therefrom; I

FIGS. S- a to 8-a and FIGS. S-b to 8-b are wiring diagrams of apparatus for obtaining the asymmetric corona discharge, as used in the present invention, and graphs showing the wave forms of the asymmetric corona discharge currents obtained therefrom; and

FIG. 9 is a schematic view, of an apparatus for practicing the process of the present invention. 1

The overcoated photosensitive member usedin the invention consists "of either three or four layers; The

photo sensitive material shown in FIG- l-a is-made up of a conductive base 1, atop which is superposed a'photoconductive layer 2 and a surface-insualting layer 3. Instead of providing a single insulating layer, an intersistance of the photoconductor, the surface of the phobinders, such as water glass, resinous binders are useable. The resinuous binders, which can be used, include such as a styrene polymer or copolymers thereof, polyvinyl acetate or copolymers thereof, acrylic resins, pol yvinyl acetals or'their copolymers, polyvinyl alcohol, polyolefins or copolymers thereof, alkyd resins, polyester resins, silicone resins, epoxy resins and synthetic rubbers. Suitable binders are disclosed in British Pat. No. 1,020,506. I

To increase the sensitivity of the photoconductor to a light source the photoconductor can be treated with an optional sensitizing dyestuff such as rose be'ngal or methylene blue. Again, the photoconductor may be used after activating with a metal suchas gold or copper. In addition, to improve such properties as the preexposingeffects (light fatigue effect) and the dark retoconductor can be treated with a Lewis acid, fatty acid or metal salts thereof and organic phosphoric acid ester compounds. I As the conductive base, which supportsthe photoconductive layer, metallic base plates of such as aluminum or copper, as well as metallic vapor deposited or plated resins, conductiveresins, papers applied a hydroscopic salt or a conductive substance, and base plates applied a composition of a metallic powder and I a resin can beused. The base plate may be either a flat mediate-insulating layer can be disposed .inside the photoconductive layer or between the photoconductive layer and, the conductive base. For example, the sensitive material may be as shown in FIG. l-b, i.e., an overcoated assembly consisting. of a conductive-base 1, a first photoconductive layer 2,an intermediate insulating layer 3, a second'photoconductive layer '2 and a surface insulating layer 3; or as shown FIG. l-c, i.e., aovercoated assembly consisting of a conductive base 1, an intermediate insulating layer 3, a photoconductive layer 2 and a surface insulating layer 3. v

The photoconductive layer is that which is known per se and any may be used as desired. For instance, a phtooconductor or a combination of a photoconductor and a binder can be used as the photoconductive layer. The photoconductor are such as inorganic photoconductive includes material materials such as selenium, zinc oxide, cadmium sulfide, zinc cadmium sulfide, cadmium telluride, (DdTe), selenium telluride (SeTe), cadmium selenide (CdSe) and antimony trisulfide (Sb S and such organic photoconductive materials as anthracene, anthraguinone and polyvinyl carbazole. These photoconductors can be used per se as the photoconductive layer. For instance, selenium or cadmium sulfide can be vapor deposited on a suitable base, or a resin which initself possesses photoconductive properties, e.g., polyvinyl carbazole, can. be applied to a base to form the photoconductive layer. 4

Alternatively, a photoconductor can be first dispersed in a binder and'then this can be applied to a conductive base. As the binder, in addtion to inorganic plate or one which is cylindrical in shape.

As the insulating layer, the transparent dielectrics having high dielectric strength such, for example, as the films of polyesters, cellulose esters, polystyrene and.

above the surface insulating layer (FIG. 2-a: first charge). In FIG. 2-a the surface of the surface insuIat-. ing layer 3 becomes charged up topotential 6 due to a charge of 'a given polarity (e.g., positive). On the other hand, the interface portion between the photoconductive layer 2 and insulating layer 3 becomes charged up to a potential 7 due to a charge of opposite polarity (negative). When the photoconductor is a negative type semiconductor in this case, the surface of the I photoconductive layer is ,applied a positive electrostatic charge by applying the electrode with a positive 7 discharge voltage. On the other hand, when the photoconductor is a positive type semiconductor, .the surface of the photoconductive layer is applied a negative electrostatic charge by applying the electrode with a negative discharge voltage. While the corona discharge voltage of the first charge varies depending upon the length of the discharging path as well as the hind of the photoconductor, generally speaking, voltage ranging between about 5,000 and about 10,000 volts is preferred.

Next, the photosensitive member, which has been applied the first charge, is applied a charge of a corona discharge current of an asymmetric wave form from a charging apparatus 9 having an electrode 10; at the same time the photosensitive member is exposed to an image pattern of lightand shadow (FIG. 2-5: deemed charge-image exposure). While the corona discharge current of an asymmetrical wave form and the exposure are preferably conducted at the same time, it need not be necessarily at the same time in the case whre the change from'the light resistance to the dark resistance of the photoconductor used as the photosensitive layer is not rapid. "Inthis case'the corona discharge current of an asymetrical wave form'may'be applied immediately after the exposure. The charge by means of the corona discharge current of an asymetrical waveform, asherein used, must be onein which the discharge current of a polarity opposite that of the first charge is greater than that of the same polarity and moreover must be one which has a degree of asymmetry sufficient to change the'polarities of the potentials of both the light and shadow portions of the image pattern of the photosensitive member to opposite polarities as well as to form an electrostatic latent image of the dark area v The so preparedsensitive material having electro static latent images in 'which 1 the polarities of the charges of the light image portion and the dark image portion differ from each other can be then developed by methods which per se are known! For instance, in developing the charged image (6') of the dark image portion (D) such'as of FIG. 2-d, development by means of the same polarity as the first charge and an electrostatic latent image ofthe lightarea of a-polarity opposite that of the first chargeduring the uniform illumina- I tion over the entire surface area of. the photosensitive member.-As a result ofthis secondcharge and theexposure, at thelight image portion (L) the photoconductive layer 2 becomes conductive'and the charge at the interface of the surface insulating layer 3 and the photoconductive layer 2 is caused to disappear to result 7 inthe potential becoming equalto the potential of the conductive base (ground potential), and at the same time, due to the asymmetric current charge the surface of the insulating layer 3 is charged with a polarity (negative).opposite that of the first. charge to result in a-new potential 6" On the other hand, at the dark image portion (D), due to the photoconductive layer 2 being nonconductive, the charge at the interface of the photoconductive layer and the surface: insulating layer formed by the first charge remains, and'therefore the' potential 7 is maintained and .the charge of the surface of the insulating layer formed by the first charge'being .of the use of toner particles (6' t) having a charged polarity opposite that of the charge of the dark portion (D) will do. On the other hand, in developing the charged image (6) of the light image portion (L), development by means of the use of toner particles (6"!) having a charged polarity opposite that of the light image portion (L) willdo. Thus, in'accordance with the invention process, the negative and positive images can be developed by choosing, as required, toners either positively or negatively charged. In addition, since the background of the image is charged with a polarity opposite that of the latent electrostatic image to be developed, the toner particles receivesimultaneously an attractive force of the staticsurfacelcharge of opposite polarity and a repulsive force of the static surface charge of the same polarity to make it possible to carry" out the development with greater efficiency. As a result, there is no soiling of thebackground.

As the developer, such developers as the powder developer, liquid developer and mist developer are usable. Again, any of the developing procedures can be slightly neutralizedby the corona discharge current of asymmetrical wave form becomes a new potential 6'.

Thus, the polarity of the surface potential of the photosensitive memberlbecomes as a whole opposite that of the first charge.

vNext, when this sensitive material is exposed to uniform illumination over the entire surface area with an activating radiation-from a light source 1 l, thee shadow image portion D of the photoconductive'layer 2 of the photosensitive member also becomes a conductive layer, and the interfacial potential 7 between, the photoconductive layer 2 and the insulating layer 3 becomes equal to the-potential of the conductive base (ground potential). While it is preferred to carry out the uniform illumination over the entiresurface of the photosensitive member in this step, similar results can be obtained by not performing this uniform illumination of the photosensitive member but by allowing the charge of the foregoing interfare to disappear by providing a sufficient period of time before'subrnitting the photosensitive member to the development step which follows, or by carrying out the development in an anbient light. Thus, there is formed, along with the charges and the potentials resulting therefrom a latent electrostatic image of the light portion (6", L) of apolarity opposite that of the first charge and a latent electrostatic image of the dark portion (6', D) of the same polarity asthat of the first charge (FIG. 2-0: overall uniform illumination).

the photosensitive member proceeds through the sev- I eral steps are shown in FIG. 3-a to 3-0.v FIG. 3-a shows the surface potential of the photosensitive member as awhole, and FIG. 3-b and 3-c show respectively the potentials of the insulating layer surfaceand the potential at the interface'of the photoconductive layer and the insulating surface layer. When the photosensitive member is applied with a direct current corona discharge of a given polarity by the first charge (I), the potential of the insulating layer surface reaches a and that of the I photoconductive layer and the surface layer interface reaches a", while the overall a=a+a". Next, when simultaneously with the exposure the corona discharge current of asymmetrical wave form (II) is carried out, the potential of the photoconductive layer and the surface layerinterface at the shadow image portion (D) decays slightly by means of the dark decay and reaches .b"D. On the other hand, the potential of'the insulating surfacelayer decays as a result of the corona discharge current of asymmetrical wave form' and reaches bD (In this case since the chargeof the insulating surface layer resulting from the first charge is restrained by the charge of the photoconductive layer and the surface layer interface, it remains the same polarity as that of the first charge even after charging with the asymmetric discharge current). Thus, the surface potential of the photosensitive member as a whole becomes bD=b'D+b"D, a polarity opposite that of the first charge.

potential reaches In contrast, in the case of the light image portion (L), the potential of the photoconductive layer and surface layer interface, due to the light decay, becomes equal to that of the conductive base, i.e. zero potential (b"L), while the potential of the insulating surface rapidly diminishes as a result of the corona discharge current of the asymmetrical wave form to become a potential of a polarity opposite that of the first charge (b'L), with the consequence that the surface potential of the light image portion as a whole becomes bL=b'L+b"L.

Finally, when the photosensitive member is submitted to a uniform illumination over the entire surface area, the potential of the insulating layer surface does not change, but the potential of the potoconductive layer and the surface layer interface of the dark portion D becomes equal to the ground potential (c"D) due to the light decay, and the surface potential of the photosensitive member as a whole becomes at the light portion (L) a potential cL, which is opposite in polarity to that of the firstcharge, while at the dark portion (D) becomes a potential cD, which is the same polarity as that of the first charge.

Thus, the use as the corona discharge for the second charge according to the new method of this invention of corona discharge current of the asymmetrical wave form in which the discharge current of a polarity opposite that of the first charge is greater than that of the same polarity is entirely different from the case where 1 a direct current or an alternating current corona discharge is used as the second charge. That is, when a corona discharge current of the asymmetrical wave form is applied, the charge at the interface of the photoconductive layer and the surface insulating layer at the light image portion readily disappears by exposure to light, with the consequence that the potential of only the insulating layer surface, as compared with the case where a normal alternating current corona discharge is applied (in this case decay to only zero potential takes place), rapidly reaches saturation at a given value of opposite potential, which value is determined by the submitted to a uniformillumination over the entire surface at this time, the aforesaid charge at the dark portion is caused to disappear by illumination and the potential of the interface and the ground potential are equalized,.with the consequence that an image portion corresponding to the exposed portion (L) and an image portion corresponding to the unexposed portion (D) of the photosensitive member can be easily formed as latent electrostatic images having polarities different from each othenln the development step, which follows, it becomes possible to obtain either a negative or positive image by the mere choice of the polarity of the toner particles. In either case clear images of high contrast and a minimum of fogging, i.e., whose background is clean, can be obtained.

degree of asymmetry of the asymmetric corona dis- The corona discharge current of asymmetrical wave form to be used in the invention process can be produced by an optional method. FIG. 4-a shows a-conventional alternating corona discharge current generating apparatus. When the primary'side 14 of the high voltage transformer is connected'to a commercial altemating current source and the two terminals of the secondary side 12 are connected to the electrode 10 and the opposite electrode 13 of the corona generating apparatus 9, an alternating corona discharge current is produced between the electrode 10 and the opposite electrode l3. Thee current wave form of this corona discharge, as shown in FIG. 4-b, is of normal wave form. The present invention uses a corona discharge which differs from this normal wave form discharge and is as corona discharge current of asymmetrical wave form in which a discharge current of a given polarity is greater than the discharge current of opposite polarity. To accomplish this, the wiring-is as shown in FIG. S-a. One terminal of the high voltage transformer is connected to the corona discharge electrode 10 through the intermediary of a rectifier 15 and a resistor 16 connected in parallel, and the other terminal of the transformer'is connected to the opposite electrode 13. The wave form of the corona discharge current obtained by such a wiring becomes one in which, for example the discharge current of the positive side is restrained while that of the negative side is substantially not restrained, i.e., an asymmetric wave fonn. In this case the magnitude of the discharge current of a given polarity can be freely regulated by changing the value of the resistance 16 connected in parallel with the rectifier l5, and the degree of asymmetry of the wave form of the current can be easily regulated by adjusting the value of the resistor. Altemately, the discharge current of a given polarity can be restrained, as shown in FIG. 6-a, by providing a controlling grid 17 between the corona discharge electrode 10 and the opposite electrode 13 applying the controlling grid with a direct current voltage from a direct current source 18. In this case also thewave form of the'current of the corona discharge becomes asymmetric,'as shown in FIG. 6-b, and the degree of asymmetry of the wave form of the current can be regulated by adjusting the voltage that this controlling grid 17 is applied with. Further, as. shown in FIG. 7-a, instead of providing a controlling grid between thecorona discharge electrode 10 and the opposite electrode 13, an shield 9 which upper side is optically opened disposed in proximity to the discharge electrode 10 can be connected to a direct current voltage source 19, and by applying this shield with a direct current voltage the corona discharge of a given polarity can be restrained, with the consequence that an alternating current corona discharge. current having an asymmetric wave form, as shown in FIG. 7-b, can be obtained.

Further, as shown in FIG. 8a, one terminal of the sec? ondary side 12 of the high voltage transformer can be grounded through the intermediary of a direct current voltage source 20, while the other terminal of the transformer is connected to the corona discharge electrode 10 and the opposite elelctrode 13 is grounded. As a re- FIG. 8-b, is obtained.

In all the foregoing cases, as the alternating current one ranging between 5,000 and 10,000 volts'is conveniently used. The degree of asymmetry'of the corona discharge current, of asymmetrical'wave form used in a cylindrical conductive base member 1. This sensitive drum rotates in the direction of the arrow and in so doing successively arrives at the several treatment zones disposed aboutthe periphery. of the drum. The

conductive base mernber 1 is grounded. First,.the drum surface 3 receives either a positiveor. negative direct current corona discharge from a corona discharge apparatus having a discharge electrode 5 connected to a direct current source (a). Next, the drum surface is ex-' posed to an image pattern by means of the exposing device 8 disposed next to the electrode 5 and-at the same time receives a corona discharge current of asymmetrical wave fonn from a-corona discharge apparatus 9 having adischarge electrode 10 connected to corona discharge currentsource of asymmetrical wave form. This is followed by an' uniform illumination all over the surface area of the drum surface by means of a light source "11 to form on the drum surface latent electrostatic image of light and dark whose polarities differ from each other. This latent electrostatic image is developedby means of a magnetic brush 23 disposed inside of a developing device 22 with a toner 6 t'which has been charged with a charge opposite that of the latent electrostatic image whose development is desired. A transfer sheet'24 is fed to between .aroll 25 and the sensitive drum and pressed against the drum surface by means of roll 25, and the transfer of the toner image take place at this point. The transfer sheet 24 to which has beentransferred the toner image is conveyed to a fixing device 26 where the toner image is fixed. Thus, a copy of either a positive-image or a reversed image can'be obtained as desired with a sharp contrast and no such defects as fogging. Next, the photosensitive drum is removed of its charge by means of a corona discharge device having a discharge electrode 28connected to an alternating current source C and cleaned at a cleaning apparatus 29 equipped with a toner removing brush 30, after which the reproduction operation is repeated.

The following exampleand controls are given for fur ther illustration of the invention.

EXAMPLE 1 4 l0 grams of an acrylic resin are added to 90 grams of foregoing sensitive plate is then applied a corona discharge of +7KV to positively charge the Lumilar surface uniformly. Next, a 450-watt tungsten lamp is placed at a pointone meter from the sensitive plate, and the surface of the plate is exposed through an optical system to anoptical image for about one second. Simultaneously with the exposure to the optical image corona discharge current of asymmetrical wave fonn is applied to .form a latent electrostatic image in the Lumilar surface. The asymmetric corona discharge current of asymmetrical wave form is obtained in the case using agenerating apparatus wired as shown in FIG. 7-1 and by applying the corona discharge elec-' trode with an alternating current voltage of 8,000 volts and the open shield with 500 volts, while regulating the ratio of negative corona discharge to the positive corona discharge to a ratio of 3:1. After the formation of the latent electrostatic image, the plate is brought into an ambient light and developed with a positively charged toner, and as a result a good quality negative reproduction image without fogging and of small edge effect is obtained. Further, when a latent electrostatic I image produced in similar manner is developed with a negatively'charged toner, a good positive reproduction image is likewise obtained.

CONTROL I .A sensitive plate prepared by the procedure describedin Example 1 is applied a +7,000V corona dis-' charge, audits Lumilar surface is positively charged. Next, a 300-W tungsten lamp is placed at a point one meter from this sensitive plate, and the surface of the plateis exposed for about one second to an optical image throughan optical system. Simultaneously with this exposure to'the optical image a normal alternating current corona discharge is applied to the plate and a plate, and the plate is exposed for about one second to copper-activated cadmium sulfide followed by the additiori of a small quantity of a solvent.and thorough 1 forming a photoconductive coating on the paper. Next,

a Lumilar (A polyethylene terephthalate film) film of about l2-micron thickness is superposed to the foregoing photoconductive coating surface with a binder to thus obtain a sensitive plate. The Lumilar surface ofthe CONTROLL i The sensitive plate of Examplel is applied a +7,000V corona discharge, and the Lumilarsurface is positively charged uniformly. Next, a 450-W, tungsten lamp is placed at a point one meter from this sensitive an optical image through an optical system. Simulta neously with this exposure to the optical image a -7KV direct current corona dischargeis applied and a latent electrostatic image is formed in the Lumilar surface. The plate is then brought out to an ambient light and developed with a negatively charged toner, and as a result a'positive reproduction image having an intense edge effect and of a poor quality usually characteristic of repulsion development is obtained. On the other hand, when the plateis developed with a positively charged toner, a foggy negative reproduction image is obtained.

We claim: 1. An electrophotographic a. applying a first charge by means of direct current corona discharge of a specific polarity to an overcoated photosensitive member selected from the group consisting of a three-layered overcoated sen sitive member consisting of a conductive base, a

that a latent electrostatic image of the dark area of the same polarity as the first charge and a latent electrostatic image of the light area of an opposite polarity to that of the first charge are formed during the subsequent uniform illumination over the Photoconductive layer and a Surface insulating entire surface of the photosensitive member; and

layer, and a Q f l/ overcoaFed photosensitive c. thereafter exposing said photosensitive member to member conslstlng of a conductwe base a Photo a uniform illumination over the entire surface by conductwe layer a Surface insulating layer and an means of an activating radiation to form atlatent intermediate insulating layer disposed either within electrostatic image of the light area and a latent the photoconductwe layer or between the photo electrostatic image of the dark area, said latent conductive layer and the conductive base;

electrostatic images being of opposite polarity to b. applying to the so charged sensitive member a seceach other ond char e b means of a corona dischar e current v g y g 2. The process of claim 1 wherein the corona voltage of asymmetrical wave form, said corona discharge being one in which the ratio of the discharge cup of said corona discharge current of asymmetrical wave form is 5,000 10,000 volts.

rent of a polarity opposite that of the first charge 7 (ii) to the discharge current of the Same polarity 3. The process of claim 1 wherein said ration of 1 :2

ranges between 5:1 and 2:1.

2 an c w 1 dv 9 and i ul neously exposing said sensitive member through an 4. The process of claim 1 wherein said overcoated photosensitive member comprises said three-layered original having an image pattern of light and H shadow, the degree of asymmetry of said corona ovemoated photosensltwe member- 5. The process of claim 1 wherein said overcoated discharge current of asymmetrical wave form being of such sufficiency that the charged potentials of photosensitive member comprises said four-layered the light and dark areas of the photosensitive memovercoated photosensitive member. ber are both transformed to opposite polarities and 

2. The process of claim 1 wherein the corona voltage of said corona discharge current of asymmetrical wave form is 5,000 -10,000 volts.
 3. The process of claim 1 wherein said ration of i1:i2 ranges between 5:1 and 2:1.
 4. The process of claim 1 wherein said overcoated photosensitive member comprises said three-layered overcoated photosensitive member.
 5. The process of claim 1 wherein said overcoated photosensitive member comprises said four-layered overcoated photosensitive member. 